An example of conventional beverage extraction apparatus is that disclosed in Japanese Utility Model Laid Open No.Sho61-118526. As shown in FIG. 1, beverage extraction apparatus 100 is provided with cylinder 101 supported to be movable up and down to which a mixture of a powdered raw material and hot water is supplied from upward and discharged downward, filter 102 for filtration of the mixture discharged from cylinder 101, beverage receiver 104 having rubber seal 103 to receive the beverage extracted by filtration with filter 102, beverage supply tube 105 for supplying the beverage to a cup, cam system 106 consisting of pressure plate 106a and cam 106b, and a control system (not shown) to control each par of beverage extraction apparatus 100. Filter 102 is placed under cylinder 101. Beverage supply tube 105 is connected with beverage receiver 104.
Cylinder 101 is provided with mixing chamber 101b for mixing the powdered raw material and water supplied from inlet 101a placed in the upper part thereof and extraction chamber 101d in which the beverage is extracted and discharged from outlet 101c placed in the lower part thereof. Mixing chamber 101b is communicated with extraction chamber 101d by way of conduit 101e, which is opened or closed by means of valve 107. Air pipe 109 is connected with extraction chamber 110d in order to introduce compressed air into extraction chamber 101d from air pump 108.
In beverage extraction apparatus 100 thus constructed, the control unit, not shown, drives an extraction driving motor (not shown) according to serving signal inputted therein, thereby cylinder 101 is lowered from stand-by position P and its lower end is brought into contact with rubber seal 103 on beverage receiver 104. Thereafter, the powdered raw material and hot water are supplied from inlet 110a. The control unit drives air pump 108 to supply compressed air to extraction chamber 101d through air pipe 109. In this state, valve 107 is opened a little, thereby, the compressed air is supplied from extraction chamber 101d through conduit 101e to mixing chamber 101b, so as to agitate the mixture of powdered raw material and hot water supplied to mixing chamber 101b. When the mixture is stirred sufficiently, air pump 108 is allowed to stop and the mixture flows into extraction chamber 110d through conduit 110e.
FIG. 2 shows the extraction process. The control unit drives the extraction driving motor to elevate valve 107 so as to close conduit 101e, and drives also air pump 108 to supply compressed air to extraction chamber 110d through air pipe 109. The mixture stored in extraction chamber 101d is filtered by filter 102, being forced by the compressed air. The pressure applied to beverage supply tube 105 by pressure plate 106a is removed by the rotation of cam 106b in a predetermined angle according to driving of a serving motor (not shown), thereby beverage supply tube 105 is opened.
When the beverage filtered by filter 102 is supplied to a serving cup (not shown) through beverage supply tube 105, cylinder 101 is moved to waste disposal position P' shown in FIG. 2, filter 102 is removed from beverage receiver 104, and the waste left on the filter 102 is discarded, being controlled by the control unit. The extraction process is thus finished. The control unit drives extraction driving motor so that cylinder 101 returns to position P.
In such a beverage extraction apparatus in which the mixture of powdered raw material and hot water is extracted under air pressure, time for filtration varies depending on the resistance on passage of the water through the filter, kind of the raw material, fineness of the powder, and quantity of the raw material.
FIGS. 3A to 3D show the change of pressure within the cylinder during filtration in the beverage extraction apparatus. FIG. 3A shows the pressure change in the case where a predetermined amount of the powdered raw material is used. After the filtration is started at time A, the pressure within the cylinder is elevated. At time B when most of the liquid component in the cylinder is filtered, the pressure decreases. After air blowing is conducted for a predetermined period for preventing the extracted beverage solution from remaining in the beverage supply tube, pressure in the cylinder is removed at time C. FIG. 3B shows the change in pressure when the amount of the raw material is increased. The time B when most of the liquid component is filtered is delayed compared to the time B shown in FIG. 3A. Moreover, the pressure is increased because the waste of powdered raw material deposited on the filter after filtration is increased to have a greater thickness.
With respect to a beverage vendor provided with the beverage extraction apparatus mentioned above, there are two modes of extraction, of which one is timely extraction in which the beverage is extracted in cups one by one on demand of service and the other is batch extraction in which a lot of beverage is extracted once and stored in a tank. In batch extraction, the powdered law material and hot water come into contact more gently than in timely extraction. Thus, a beverage of better quality having less bitter and less impure taste can be extracted. In batch extraction, a beverage of high quality can be served quickly even when the serving is very busy because the beverage stored in the tank is served on demand of service.
FIG. 3C shows the change in pressure in the cylinder in the case of batch extraction. Low pressure of air is applied to the mixture of a powdered raw material and hot water to filter it in a long period, thus, time B when almost all of the liquid component is filtered is delayed compared to time B shown in FIG. 3B.
As described above, the time required for filtration varies depending on the manner of extraction. In an arrangement, for example, in which filtration is terminated by a timer, it is necessary to set a time for filtration in slight excess so that the filtration is complete in substance when the time set for filtration is over. The time for complete filtration may deviate from the time set by the timer, due to the changes in the amount of powdered raw material and hot water, pressure loss in pipings, fluctuation of the air pump and so on. If the filtration time is too short, filtration is not enough. If the filtration time is too long, compression by air is continued on the waste of the raw material from which the liquid component has been filtered out, resulting in extraction of astringency, bitter and other useless components.
A beverage extraction apparatus free from such obstruction is disclosed, for example, in Japanese Patent Laid Open No.Sho63-12088. In this beverage extraction apparatus, filtration is terminated by detecting the change in pressure within the hot water-supplying cylinder according to the finishing of filtration by means of a pressure sensor. Other arrangements of beverage extraction apparatus are disclosed in Japanese Patent Laid Open No.Sho63-47899 and Japanese Patent Laid Open No.Hei4-188396, in which the change in pressure within the hot water-supplying cylinder is detected by a pressure sensor. Various measures have been taken against defective filtration so far.
In the beverage extraction apparatuses described above, the mixture of a powdered raw material and hot water is agitated by compressed air in order to accelerate extraction to accomplish efficient extraction of the beverage on demand of service, but degrees of dissolution of the components of beverage, such as taste components, astringent components, bitter components and sour components, vary according to the intensity of agitation. The rate of extraction of the beverage can be increased by more vigorous agitation, which accelerates simultaneously the dissolution of undesirable components such as an impure taste.
A beverage extraction apparatus solving such problem is disclosed in Japanese Patent Laid Open No.Hei9-147229. In this beverage extraction apparatus, compressed air in an amount in accordance with the degree of extraction is supplied to agitate the powdered raw material and hot water, and thereafter the air is supplied to the extraction chamber to filter the mixture.
In a conventional beverage extraction apparatus in which the pressure within the water supplying cylinder is detected to terminate the filtration, the pressure within the water supplying cylinder tends to vary depending on the manner of extraction, so that no definite pressure difference is produced during the filtration and even after completion of the filtration sometimes in the case of low pressure extraction, such as batch extraction. Therefore, the completion of filtration is not determined, causing extraction of bitter and astringency. In a conventional beverage extraction apparatus in which the air pressure during the agitation is adjusted, the pressure in the later period of filtration is elevated too high because of the arrangement to increase the air pressure as time passes, whereby defective filtration sometimes occurs.